1/* SPDX-License-Identifier: GPL-2.0-only */
2#ifndef _LINUX_RANDOMIZE_KSTACK_H
3#define _LINUX_RANDOMIZE_KSTACK_H
4
5#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
6#include <linux/kernel.h>
7#include <linux/jump_label.h>
8#include <linux/percpu-defs.h>
9
10DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
11 randomize_kstack_offset);
12DECLARE_PER_CPU(u32, kstack_offset);
13
14/*
15 * Do not use this anywhere else in the kernel. This is used here because
16 * it provides an arch-agnostic way to grow the stack with correct
17 * alignment. Also, since this use is being explicitly masked to a max of
18 * 10 bits, stack-clash style attacks are unlikely. For more details see
19 * "VLAs" in Documentation/process/deprecated.rst
20 *
21 * The normal __builtin_alloca() is initialized with INIT_STACK_ALL (currently
22 * only with Clang and not GCC). Initializing the unused area on each syscall
23 * entry is expensive, and generating an implicit call to memset() may also be
24 * problematic (such as in noinstr functions). Therefore, if the compiler
25 * supports it (which it should if it initializes allocas), always use the
26 * "uninitialized" variant of the builtin.
27 */
28#if __has_builtin(__builtin_alloca_uninitialized)
29#define __kstack_alloca __builtin_alloca_uninitialized
30#else
31#define __kstack_alloca __builtin_alloca
32#endif
33
34/*
35 * Use, at most, 6 bits of entropy (on 64-bit; 8 on 32-bit). This cap is
36 * to keep the "VLA" from being unbounded (see above). Additionally clear
37 * the bottom 4 bits (on 64-bit systems, 2 for 32-bit), since stack
38 * alignment will always be at least word size. This makes the compiler
39 * code gen better when it is applying the actual per-arch alignment to
40 * the final offset. The resulting randomness is reasonable without overly
41 * constraining usable stack space.
42 */
43#ifdef CONFIG_64BIT
44#define KSTACK_OFFSET_MAX(x) ((x) & 0b1111110000)
45#else
46#define KSTACK_OFFSET_MAX(x) ((x) & 0b1111111100)
47#endif
48
49/**
50 * add_random_kstack_offset - Increase stack utilization by previously
51 * chosen random offset
52 *
53 * This should be used in the syscall entry path when interrupts and
54 * preempt are disabled, and after user registers have been stored to
55 * the stack. For testing the resulting entropy, please see:
56 * tools/testing/selftests/lkdtm/stack-entropy.sh
57 */
58#define add_random_kstack_offset() do { \
59 if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
60 &randomize_kstack_offset)) { \
61 u32 offset = raw_cpu_read(kstack_offset); \
62 u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset)); \
63 /* Keep allocation even after "ptr" loses scope. */ \
64 asm volatile("" :: "r"(ptr) : "memory"); \
65 } \
66} while (0)
67
68/**
69 * choose_random_kstack_offset - Choose the random offset for the next
70 * add_random_kstack_offset()
71 *
72 * This should only be used during syscall exit when interrupts and
73 * preempt are disabled. This position in the syscall flow is done to
74 * frustrate attacks from userspace attempting to learn the next offset:
75 * - Maximize the timing uncertainty visible from userspace: if the
76 * offset is chosen at syscall entry, userspace has much more control
77 * over the timing between choosing offsets. "How long will we be in
78 * kernel mode?" tends to be more difficult to predict than "how long
79 * will we be in user mode?"
80 * - Reduce the lifetime of the new offset sitting in memory during
81 * kernel mode execution. Exposure of "thread-local" memory content
82 * (e.g. current, percpu, etc) tends to be easier than arbitrary
83 * location memory exposure.
84 */
85#define choose_random_kstack_offset(rand) do { \
86 if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
87 &randomize_kstack_offset)) { \
88 u32 offset = raw_cpu_read(kstack_offset); \
89 offset = ror32(offset, 5) ^ (rand); \
90 raw_cpu_write(kstack_offset, offset); \
91 } \
92} while (0)
93#else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
94#define add_random_kstack_offset() do { } while (0)
95#define choose_random_kstack_offset(rand) do { } while (0)
96#endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
97
98#endif
99